The invention relates to an operating circuit for a low-pressure discharge lamp.
Low-pressure discharge lamps have lamp electrodes, as a rule two electrodes per lamp, that have a limited service life. The end of the service life of the lamp is generally given by the end of the service life of an electrode.
It is known that low-pressure discharge lamps should be replaced if at all possible when the failure of an electrode is imminent. The reason for this is chiefly that shortly before the end of the service life of an electrode there is an unusually high electrode drop that leads to high temperatures of the electrode and of the neighboring region of the discharge lamp. This can result in safety problems, above all in the case of small low-pressure discharge lamps and heat-sensitive installation situations.
Use is made for this purpose of detection circuits for detecting the end of the service life of the electrodes (end-of-life detection: referred to below as EOL detection, for short). One known option for early EOL detection consists in measuring the voltage across a so-called coupling capacitor that connects an electrode to the positive or negative terminal of the supply and decouples the lamp in DC terms and couples it in AC terms to the supply. This coupling capacitor is charged in normal operation on average over time to half the supply voltage. Deviations from this value can be sensed by a comparator and used for detecting an impending end of service life.
This optional solution has proved to be disadvantageous with regard to accuracy and technical outlay.
Starting therefrom, the invention is based on the technical problem of specifying an operating circuit for a low-pressure discharge lamp with an EOL detection circuit that is simple and permits reliable and safe operation of the lamp.
Provided according to the invention for this purpose is an operating circuit in which the EOL detection circuit can measure the DC voltage between the electrodes in order to carry out the early detection with the aid of the measured DC voltage, and the DC voltage between the electrodes can be modified by an offset voltage such that only one polarity occurs when measuring the modified DC voltage between the electrodes by means of the EOL detection circuit.
The particular feature of the operating circuit according to the invention resides in the fact that the EOL detection circuit now measures the DC voltage between the electrodes of the low-pressure discharge lamp. Given completely intact electrodes, ideally no DC voltage occurs during operation. It should be recalled here that the low-pressure discharge lamp is operated solely with the aid of alternating current and is decoupled in DC terms from the operating circuit.
However, it has emerged that a DC voltage results with increasing electrode degeneration by virtue of the fact that a somewhat more pronounced electrode drop zone is formed in front of the electrode which is likely to have the shorter service life. The low-pressure discharge lamp therefore has a rectifying effect overall. This asymmetry is increased by the advancing aging of the electrode with the shorter service life up to its failure. A voltage threshold for which the early detection of expected failure of an electrode takes place can be established empirically.
The advantage resides in the measurement of comparatively low voltages that can be processed with the aid of semiconductor components without the need for excessively high voltage divider ratios. Specifically, voltage divider circuits with high division ratios are always associated with accuracy problems that can be resolved only by a costly selection of components. In addition, the inventive mode of procedure of directly measuring the DC voltage between the electrodes is particularly simple and scarcely dependent on further details of the operating circuit.
The invention also provides that this DC voltage measurement is performed such that an offset voltage produces a shift in the DC voltage, effected by the processes in the interior of the lamp, between the electrodes such that finally only voltage values of one polarity occur in the range of the DC voltages which is permissible before the early EOL detection. It is thereby possible to simplify considerably the design of the measuring device used for the DC voltage measurement.
Furthermore, it is also possible to make use in the invention of a voltage divider circuit for dividing the DC voltage to be measured between the electrodes, a tapping point being provided for the EOL detection circuit. By contrast with the prior art described, however, the invention requires detection between the electrodes of only comparatively low voltages that necessitate a very much weaker voltage division than the half supply voltage, used conventionally, at the so-called coupling capacitor. Thus, the sensitivity to faults in the components used is also lower.
According to the invention, these advantages may be associated with the fact that the EOL detection circuit has an electrode interrogation function. The safety advantage already achieved for the operating circuit by early EOL detection can be further enhanced by the electrode interrogation function. Specifically, the electrode interrogation determines whether the terminal or terminals of a holder, connected to the operating circuit, for the low-pressure discharge lamp is/are connected to the associated electrode. If no electrode is present, the low-pressure discharge lamp is not correctly inserted or is defective. If no electrode is present, presumably no discharge lamp has been inserted at all, and this gives rise to the need to prevent the application of high voltage to the holder in order to exclude danger to persons.
The electrode interrogation function is performed by virtue of the fact that the EOL detection circuit can sense a reference potential via the respective electrode. If the connection to the reference potential is lacking, this is sensed by the EOL detection circuit, the result being information about the presence of the electrode.
Advantages result even if only one electrode can be interrogated in the way described. This is because even at this stage the safety aspect of preventing voltage from being applied in the event of a missing discharge lamp occurs. In particular, it is possible in this case to interrogate an electrode xe2x80x9cnearer to groundxe2x80x9d, because contacting the electrode xe2x80x9cremote from groundxe2x80x9d would be less dangerous (interrogation of the xe2x80x9ccold endxe2x80x9d).
However, an interrogation of all the existing electrodes is advantageously provided, that is to say of two electrodes, as a rule. This gives the advantage, for example, of also being able, in any situation, to detect a defect in a discharge lamp just inserted. In the case of this embodiment, the EOL detection circuit must thus be connected to in each case a first terminal of all the electrodes, whose respective other terminal is connected to the respective reference potential.
The use of the potential of the operating circuit, serving as ground, for the or at least one of the reference potentials is, due to its simplicity, a particularly advantageous variant of the invention.
Furthermore, one embodiment provides that electrode interrogation uses the same measuring input and the same electrode taps as the DC voltage measurement for the purpose of early EOL detection.
The measurement of the DC voltagexe2x80x94possibly voltage-dividedxe2x80x94between the electrodes and the electrode interrogation function are preferably carried out via a microcontroller. Furthermore, this microcontroller can also supply an output voltage to be used to generate the offset voltage. The output of the microcontroller that is used for the offset voltage is preferably connected via a resistor to the already mentioned tapping point of the voltage divider circuit. Reference is made to the exemplary embodiment.
Furthermore, the operating circuit according to the invention can be configured such that it responds in the case of early EOL detection only when the DC voltage between the electrodes that triggers the detection has already occurred for a specific minimum time. This is because experience demonstrates that it is possible at the start of operation and also during continuous operation for short-term phenomena to arise in the discharge lamp which could trigger an early EOL detection, that is to say cause correspondingly high DC voltages between the electrodes. Such faulty detections can be prevented by defining a minimum sensing time. In the case of the microcontroller already mentioned, consideration is given, for example, to loop interrogations or averaging operations over a specific number of measured values. This time delay can be tolerated without danger because of the thermal inertia, present in any case, of the discharge lamp itself.
In addition, the operating voltage can also be designed for a plurality of discharge lamps, for example for two discharge lamps. It is then preferred to provide a series connection of electrodes of one of the discharge lamps and an electrode of the other discharge lamp. The remaining electrode can then be connected to ground. Reference may be made to the exemplary embodiment.